The invention relates to a method and an apparatus for determining and/or monitoring the quantity and quality of a fluid in which a lowering of the boiling temperature is symptomatic of a deterioration in quality. This is the case in particular with hygroscopic fluids, for example with glycol-based brake fluids or with engine oil.
Since the boiling point of brake fluid is known to be lowered by absorption of water, with the result that the serviceability of the brake is reduced if the brake fluid is strongly heated, monitoring the state of the brake fluid is particularly important. Furthermore, it is also expedient in this connection to establish the filling level of the brake fluid in the hydraulic system.
DE 36 39 664 C2 already discloses a method and an apparatus for monitoring the state or the condition of a hydraulic fluid, the boiling point being determined with the aid of sensor elements. Apart from the boiling point, the momentary temperature of the fluid is measured. As a measure of the temperature difference between the boiling point and the momentary temperature, a so-called "thermal reserve" is formed and displayed.
DE 41 13 443 C2 discloses an apparatus for detecting a liquid or gaseous medium, in particular a fluid filled to a certain level in a vessel, having at least one sensor unit which can be brought into contact with the medium. For this purpose, a heating element and a temperature sensor are used, the influencing of the temperature sensor by the heating element taking place in a way dependent on the presence or absence of fluid of the medium to be tested.